1. Field of the Invention
The present invention relates to a marine riser, of the type used in the transport of hydrocarbon fluids (gas, and/or oil, and/or water) from offshore wells.
2. Description of Prior Art
The marine riser typically includes a number of conduits for the transport of fluids and different conduits within the riser tower are used to carry the hot production fluids and the injection fluids which are usually colder. The Marine riser may form part of a so-called hybrid riser, having an upper portion (“jumpers”) made of flexible conduit and a lower portion made of rigid spools. U.S. Pat. No. 6,082,391 proposes a particular Hybrid Riser Tower (HRT™) consisting of an empty central core, supporting a bundle of riser pipes, some used for oil production some used for water and gas injection. This type of tower has been developed and deployed for example in the Girassol field off Angola. Insulating material in the form of syntactic foam blocks surrounds the core and the pipes and separates the hot and cold fluid conduits. Further background is to be published in a paper Hybrid Riser Tower: from Functional Specification to Cost per Unit Length by J-F Saint-Marcoux and M Rochereau, DOT XIII Rio de Janeiro, 18 Oct. 2001. Other forms of riser tower are described in applications WO 04/051051 and WO 04/051052.
It has been current practice to connect Riser Towers to a FSPO using flexibles in a catenary configuration. Alternatively an earlier version of the Hybrid Riser Tower was connected to a semi-submersible with flexibles in a catenary position (see Garden Banks 388, Offshore Engineer August 1994).
It is also known to attach a riser (in a “lazy wave” or “lazy s” configuration) to a vessel such as a Floating, Production Storage and Offloading vessel (FPSO) via a turret type connector attached to the top of the riser and which mates with a recess usually in the vessel's hull. The turret usually comprises a rotating table and a set of swivels. In a disconnectable turret the rotating table is released and the swivels remain on board. This then retains the vessel at a fixed place but allows it to rotate with the rotating table according to sea and weather conditions, while also allowing the fluid to pass through this connection into the vessel.
In difficult meteorological and sea conditions, particularly when in deepwater and ultradeep water in areas where typhoon or hurricanes can strike, it may be required to disconnect the floating production facilities. It is important that the load imposed on the turret when it is disconnected remains minimal. A substantially rigid riser tower such as a HRT is a suitable means of having a very small load and carrying a number of connections (lines and umbilicals) to the turret.
However, little work has been performed to attach a riser tower or HRT arrangement to a disconnectable turret or loading buoy.